1. Field of the Invention
The invention of the present application relates to a semiconductor device having a circuit which is configured of thin film transistors (hereinbelow termed xe2x80x9cTFTsxe2x80x9d), and a method of fabricating the-semiconductor device. By way of example, it relates to an electrooptical device which is typified by a liquid crystal display panel, and an electronic equipment in which such an electrooptical device is installed as a component.
Incidentally, here in this specification, the semiconductor device is intended to signify general devices which can function by utilizing semiconductor properties, and electrooptical devices, semiconductor circuits and electronic equipment are all the semiconductor devices.
2. Description of Related Art
In recent years, notice has been taken of technology wherein thin film transistors (TFTs) are constructed using a semiconductor thin film (having a thickness on the order of severalxcx9ca few hundred nm) which is formed on a substrate having an insulating surface. The TFTs are extensively applied to ICs and electron devices such as electrooptical devices, and it is especially hurried to develop them as the switching elements of an image display device.
Hitherto, liquid crystal display devices have been known as image display devices. The liquid crystal display device of active matrix type has come to be often employed because an image of: higher definition than by the liquid crystal display device of passive type can be obtained. In the active matrix type liquid crystal display device, a display pattern is formed on a screen by driving pixel electrodes arranged in the shape of a matrix. More specifically, voltages are applied between selected ones of the pixel electrodes and ones of counter electrodes corresponding to the selected pixel electrodes, whereby a liquid crystal layer interposed between the pixel electrodes and the counter electrodes is optically modulated, and the optical modulation is recognized as the display pattern by an observer.
The applications of such an active matrix type liquid crystal display device have widened, and a higher, definition, a higher aperture efficiency and a higher reliability have been more required together with the larger area of a screen size. Besides, enhancement in productivity and reduction in cost have been more required at the same time.
In the prior art, an amorphous silicon film is suitably employed as an amorphous semiconductor film for the reason that it can be formed on a substrate of large area at a low temperature of or below 300. Also, TFTs of inverse stagger type (or bottom gate type) each having a channel forming region formed of an amorphous semiconductor film are often employed.
Heretofore, a liquid crystal display device of active matrix type has been high in its manufacturing cost for the reason that TFTs have been fabricated on a substrate by using, at least, five photo-masks in accordance with photolithographic technology. In order to enhance a productivity and to enhance an available percentage, decreasing the number of steps is considered as effective means.
Concretely, it is: necessary to decrease the number of photo-masks required for the manufacture of TFTs. The photo-mask is employed for forming a photoresist pattern to serve as the mask of an etching step, over a substrate in the photolithographic technology.
Using each of the photo-masks, steps such as coating with a resist, pre-baking, exposure to light, image development and post-baking are performed, and steps such as the formation and etching of a film and further steps such as stripping off the resist, washing and drying are added as the preceding and succeeding steps of the first-mentioned steps. These steps are complicated, and have been problematic.
Moreover, since the substrate is an insulator, static electricity has been generated by friction etc. During the manufacturing process. When the static electricity is generated, short-circuiting arises at the intersection part of wirings laid over the substrate, or the TFTs are deteriorated or destroyed by the static electricity, so that display defects or degradation in an image-quality have/has occurred in the liquid crystal display device. In particular, during the rubbing of liquid crystal orientation processing which is performed in the manufacturing process, the static electricity appears and has been problematic.
The present invention consists in replying to such problems, and in a semiconductor device typified by a liquid crystal display device of active matrix type, it has for its object to decrease the number of steps for fabricating TFTs, thereby to realize reduction in a manufacturing cost and enhancement in an available percentage.
Also, it has for its object to provide a structure capable of solving the problem of the destruction of TFTs or the characteristics deterioration thereof ascribable to static electricity, and a method of fabricating the structure.
In order to solve the problems, according to the present invention, each gate wiring is initially formed by a first photo-mask.
Subsequently, a gate insulating film, a non-doped amorphous silicon film (hereinbelow, called xe2x80x9ca-Si filmxe2x80x9d), an amorphous silicon film which contains an impurity element bestowing the n-type (hereinbelow, called xe2x80x9cn+a-Si filmxe2x80x9d), and an electrically-conductive film are formed in succession.
Subsequently, an active layer, a source wiring (including a electrode) and a drain electrode which are made of the a-Si film are patterned and formed by a second photo-mask.
Thereafter, a transparent electrically-conductive film is formed, whereupon a pixel electrode made of the transparent conductive film is formed by a third photo-mask. Further, a source region and a drain region which are made of the n+a-Si film are formed, while at the same time, part of the a-Si film is removed.
Owing to such a construction, the number of the photo-masks for use in photolithographic technology can be made three.
Moreover, the source wiring is covered with the transparent conductive film which is the same material as that of the pixel electrode, thereby to form a structure in which the whole substrate is protected from external static electricity etc. It is also allowed to form a structure in which a protective circuit is formed of the transparent conductive film. Owing to such a construction, the generation of the static electricity which is ascribable to the friction between a manufacturing apparatus and the insulator substrate can be prevented during a manufacturing process. In particular, TFTs etc. can be protected from the static electricity which appears during the rubbing of liquid crystal orientation processing that is performed in the manufacturing process.